This invention relates generally to wind turbine energy sources and more particularly to the protection of marine-based wind turbine structures.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted to a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators that may be rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid.
Some wind turbine units (i.e., the wind turbine itself, the tower, support structure/foundation, auxiliary components, etc.) are installed in water or seawater, and may be installed in shallow, swift current, with shifting bathymetry and perhaps brackish water. It is known that steel structures used for marine facilities corrode, when unprotected, about 5 mils per year. Higher rates of corrosion are also possible, depending upon the composition of the water, imperfections in the steel itself, and many other factors.
To avoid or at least delay the effects of corrosion, some of these known offshore wind turbine unit installations use cathodic protection (CP) to protect wind turbine unit support structures or foundations. This technique involves the use of electrochemical reactions to prevent the corrosion of underwater steel structures. Sacrificial anodes are presently being used in such CP installations. However, if the anodes are buried by shifting bathymetry or tidal changes in water level, if the current is particularly swift, and/or the salinity of the water changes by a significant amount, the protection provided will suffer. For example, the anodes can be consumed prematurely, underprotect the structure, or passivate. Sacrificial anodes cannot adjust to changing conditions and would be subject to premature consumption or to underprotection of the wind turbine unit support structure or foundation. Locations far from the equator also have both a higher chance of annual wind force and a higher chance of higher tides. Variations in water depth of as much as 80 feet can occur due to strong tides, which can change the amount of support structure or foundation that is being corroded, along with current, waves, salinity, and bathymetry changes.
In addition, sacrificial anodes require wasteful over-design and consequential excess weights. The high weights may require the construction of stronger support structures, especially if the anodes are installed as part of the support structure, as is usually the case for platforms or tripods. Since windy offshore sites have a limited/short installation and construction season, cathodic protection sometimes cannot be installed until the next installation season, allowing initial corrosion to occur on the support structures or foundations. Furthermore, hazardous minor metal components are common in sacrificial anodes. These components are released into the surrounding water as the anodes are “sacrificed” (corroded/eroded). Some environmentalists and government bodies are concerned by the effect of this release on the surrounding environment.
In addition, a large amount of energy is consumed for the manufacture of sacrificial anodes, which are typically very heavy. In addition to increased cost, this weight results in increased safety risk both as a result of weight as well as the large number of components that must be installed.
Thus, sacrificial anode CP is not an ideal protection for wind turbine units either in regard to safety and environmental considerations, or in changing ocean conditions.